The objective of this research was the elucidation of the effect of ultrasound on the permeability of bacterial cell walls toward hydrophobic compounds. The technique of EPR was used to quantify the effect of insonation on the penetration and distribution of a lipophilic 16-doxylstearic acid (16-DS) in gram-negative Pseudomonas aeruginosa cells. A new technique was developed to quantify the intracellular uptake and distribution of 16-DS, based on the comparison of EPR spectra upon cell lysis with either 10% or 0.6% of sodium dodecyl sulfate (SDS). The former resulted in the predominant fraction of probe partitioning into the SDS micelles. In the latter case probe partitioned between SDS micelles and non-destroyed membrane fragments. The results indicate structural heterogeneity of P. aeruginosa membranes, with the presence of structurally "stronger" and "weaker" membrane sites characterized by different susceptibility to the SDS treatment.